Stepper motors are externally commutated motors commonly used for positional tasks. An open loop methodology is used to control and operate stepper motors. In this open loop methodology, a controller sends commutation signals to the motor. In open loop control it is assumed that the motor responds to the controller signals. Position is then determined from the expected motor response. For lightly loaded motors and at low speeds, the open loop methodology works well. However, in an open loop stepper motor control system, the rotor may not commutate to the desired position for several reasons, including: random electrical noise; high loads; impulse loads; and/or opens in the control lines or motor windings. These possible failures make open loop control systems undesirable in many position critical applications.
A closed loop methodology is sometimes used in systems to confirm that the stepper motor follows the commutation signals from the controller. Positional feedback can be monitored in several ways, such as adding a rotational encoder to the stepper motor shaft or by adding a linear encoder to another moving part of the system. These methods of confirming positional movement add additional complexity, require additional hardware and add additional cost to the overall system. In motor control, a back electromotive force (BEMF) voltage is used in some externally commutated motors to sense the movement of the rotor. When a motor is driven with a current, a counter electromotive force pushes against the current. In electric motors, the motor has coils that cut across a magnetic field as the coils rotate. The motor therefore acts as a generator even at the same time as it rotates as a motor. The BEMF voltage that results is proportional to the motor speed and position, and so is used to indirectly measure the motor speed and position. The typical BEMF voltage measurement process starts by idling drive transistors that supply potential to the motor, waiting a sufficient time for transient voltages to discharge, and then measuring a voltage due to BEMF. However, using these methods of measuring BEMF in a stepper motor has limitations. In a full step mode for a stepper motor, there is no time when the current in the motor windings is zero, so the BEMF voltage measurement cannot be made. In other operation modes the time available for the BEMF voltage measurement, after time for the initial magnetic field to decay, is quite short. Improvements are needed and desirable.